With continuing improvements in cost and performance, solid-state lasers have potential benefits as illumination components for display systems. Their inherent spectral purity, high brightness, and long operating life have sparked particular interest among designers of high-end color projection systems. However, proposed solutions for using laser light sources for digital projection fall short of what is needed for providing robust display apparatus that take advantage of this potential.
Various solutions that have been proposed include using laser light illumination with spatial light modulators that employ electromechanical devices. Among solutions most familiar to those skilled in the imaging arts are designs using micromirror devices, such as those using digital light projector (DLP) technology promoted by Texas Instruments, Inc. of Dallas, Tex. Another set of solutions have been proposed using grating light valve (GLV) designs, offered by Silicon Light Machines as described by in U.S. Pat. No. 6,215,579 (Bloom et al.), and others. Still other solutions have been proposed using grating electro-mechanical systems (GEMS) devices, such as those disclosed in commonly-assigned U.S. Pat. No. 6,802,613 (Agostinelli et al.) which addresses the need to represent a larger fraction of the visible color space, in electronic projection systems, by incorporating more than three primary colors of light.
Although these proposed solutions have merit, there are practical problems that have yet to be adequately addressed. DLP and other digital micromirror devices are challenged by limitations in resolution, bit-depth, yield, and scalability for large-format projection. GLV device architecture, requires a complex design of supporting components. GEMS devices, advantaged for simplicity of fabrication and capable of more compact packaging, can still require relatively complex and costly optical designs, particularly where more than three color sources are used.
One notable difficulty faced by color electronic display apparatus designers relates to the need for precise optical alignment of the individual color paths in multi-chip systems. When using multiple spatial light modulators, each spatial light modulator must be precisely in register with each other spatial light modulator. This creates difficulties for designs having more than three primary colors of light.
One workaround to this problem uses “color sequential” solutions that employ a single modulator chip and direct each primary color (typically red, green, and blue, represented as RGB) to the modulator in sequence. With such a color sequential solution, a single modulator chip is fixed in place so that each primary of the modulated light is automatically registered with respect to the other primaries. However, this advantage comes at the price of lost light, since on average only one-third of the available light from each light source can be used for a three-primary color system. Systems using four or more colors would be even more severely hampered by lost brightness.
Where it is desirable to have three or more colors, none of the existing approaches is optimal. Using three or more GEMS devices is feasible, but this type of solution makes for a more complex optical system and more difficult alignment of optical path components. Using a single chip GEMS device bypasses this alignment problem, but this solution compromises light output. As a result, even though lower cost laser light sources with increased brightness are being introduced, existing approaches to laser projection design limit the potential gains in brightness and overall efficiency.
Thus far, in spite of considerable effort and innovation, design solutions that allow use of three or more laser colors, with high efficiency, compact optics, and reduced cost have proved elusive. With the advent of high-performance and low-cost laser sources in the red, green, and blue spectra and the promise of improved color gamut in using these light sources, there is a recognized need for projection apparatus using electromechanical devices that provide high-performance, low cost laser projection display.